To provide a portable source of 120- or 240-volt alternating current (AC) at 50/60 Hz, an inverter can be connected to draw direct current (DC) from either the rectified output of a vehicle alternator or from a battery, converting the DC to AC and transforming it to the desired voltage level. A static inverter has been developed that provides improved conversion of DC to AC and includes a dual feedback control scheme. This improved static inverter is disclosed in U.S. Pat. No. 5,077,652, which is assigned to the same assignee as the present invention. The static inverter disclosed in this patent raises the DC from its nominal 12-volt level to a higher DC voltage level by first converting it to a high frequency AC, transforming the AC to a higher voltage level, and then converting the higher voltage AC back to DC, which is applied to charge a capacitor bank. The higher voltage DC stored in the capacitor bank is then converted to AC at 50/60 Hz with an H-bridge switching circuit. A tap on the transformer provides a low voltage AC source that is rectified to DC and supplied to charge the battery on a vehicle while the inverter is drawing power from the alternator. However, this approach, while more efficient than a conventional inverter, is still somewhat inefficient in using power generated by an alternator because of losses that occur in the internal rectifier diodes of the alternator. Each conversion between DC and AC adds to the inefficiency of the overall AC power supply. Thus, to improve the efficiency of a system for supplying AC to a load from a vehicle electrical system, unnecessary conversions should be eliminated.
The first conversion between AC and DC that occurs in a vehicular electrical system is in the alternator. The typical vehicular alternator includes internal rectifier diodes that convert the AC produced by the alternator into DC that is supplied to charge the vehicle's battery and provide power for its electrical system. For a three-phase alternator operating at 100 A DC output, the losses incurred by rectifying the AC with the internal diodes is approximately 350 watts (58 w/diode.times.6 diodes). Including this initial conversion that takes place within the alternator, a total of three conversions between AC and DC (or vice versa) are required to supply charging current to the battery from the prior art dynamic inverter system, when only one conversion from AC to DC should be necessary.
Conventional inverters are designed to operate with a DC input voltage and have no provision for drawing power directly from the AC produced internally by a vehicular alternator. However, there are clearly advantages in bypassing the diodes within an alternator and using the AC produced directly by the alternator as a source of power. Such a system would provide enhanced operating efficiency and be simpler, since it would eliminate the losses in the alternator diodes, which are relatively high due to the high current levels that the diodes rectify to supply power at the typical low DC output voltage of a conventional vehicular alternator. Using the AC produced by the alternator directly, without rectification, would avoid such losses. Yet there are times when it is not practical to run the vehicle's engine to drive the alternator to supply power to a dynamic inverter. For example, if an AC load driven by the inverter is relatively light, or if a prime mover used to drive the alternator can not be operated due to exhaust fumes that it would produce in an enclosed space, it would be preferable to power the AC load by inverting DC power drawn from a battery. It appears that prior art AC power supplies designed for use with a vehicle have the capability to selectively draw power either from a battery or from AC produced directly by an alternator on the vehicle. Moreover, it would be desirable for the power supply to sense the load connected and switch over from the battery source to an alternator, if the AC load exceeds a predefined level and the vehicle engine is running and available to drive the alternator.
To regulate the voltage produced by an alternator for charging a battery, a conventional voltage regulator on a vehicle senses the voltage across the battery terminals and controls the current supplied to the field windings of the alternator. However, the voltage driving the field winding current is limited to the voltage across the battery terminals, which drops as the battery is heavily loaded or discharged. When an alternator provides input power for an AC power supply, it would be preferable to provide a relatively higher DC voltage to energize the field windings to improve the efficiency of the alternator as it supplies both the power required by a connected load and the power required to recharge the battery. Accordingly, the power supply connected to the alternator should provide a source of field winding current to the regulator that is at a higher DC voltage than the field winding current normally provided by the battery.
Most conventional inverters produce a square wave or quasi-sinusoidal output wave form at 50/60 Hz. Certain loads are adversely affected by the relatively high harmonic distortion levels (45% for a square wave) present in power supplied by these inverters. It is therefor preferable to supply sinusoidal power with less than 5% harmonic distortion, to such a load, which prior art devices are incapable of providing.
The foregoing aspects and deficiencies of the prior art led to the development of the present invention, and many of the attendant advantages of this invention will become more readily appreciated as the same become better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings.